How to ask Blender to return an angle of a tangent in a driver?
The python function is atan() not arctan(), and generally its better to use atan2(y, x) instead of atan(y/x) because the former won't generate division by zero.
Also note you have the hypotense so you want asin(var_1-var_2)/6) to get $\alpha$ since you haven't actually found Y. See https://docs.blender.org/manual/en/latest/animation/drivers/drivers_panel.html#simple-expressions